A radio signal transmitted by an antenna of a communicating entity undergoes deformations according to the propagation conditions between a source point defined at the output of the source antenna and a destination point defined at the input of an antenna of the destination communicating entity. In order to limit these deformations, the signal is first distorted by applying pre-equalization coefficients according to the characteristics of the propagation channel between these two antennas. It is therefore necessary to characterize this propagation channel.
Among the existing pre-equalization methods, the methods based on time reversal stand out because of their reduced complexity, their performance and their intrinsic capacity to focus a radiowave on a receiving antenna. Time reversal makes it possible to significantly reduce the time scattering of the propagation channel by focusing the energy of the received signal in time and in space.
Time reversal is a wave focusing technique, typically for acoustic waves, which relies on the invariance by time reversal of the wave equation. Thus, a temporally reversed wave is propagated like a direct wave going back in time. A brief pulse transmitted from a source point is propagated in a propagation medium. A portion of this wave received by a destination point is time-reversed before being returned in the propagation medium. The wave converges toward the source point, reforming a brief pulse at said source point and the energy of the wave is focused on the source point. The signal, focused by time reversal on the source point, is almost identical in its form to the original signal transmitted at the source point. There is thus time recompression at the source point. In particular, the returned wave converges all the more accurately when the propagation medium is complex.
The time reversal technique is thus applied to radio communication networks to cancel the effect of the propagation channel on the signal, notably by reducing the spreading of the channel, and to simplify the processing of symbols received after passing through the channel. The signal transmitted by an antenna of the source communicating entity is thus pre-equalized by the application of coefficients obtained from the time reversal of the impulse response of the propagation channel that this signal must pass through. The time reversal of the propagation channel applied to the signal makes it possible to cancel the effect of this channel during transmission of the duly pre-distorted signal from the source point, and to focus the signal on a destination antenna. Implementing time reversal thus requires knowledge of the propagation channel by the source communicating entity.
However, when the communicating entities are mobile, the estimation of the channel made at a given instant by a communicating entity may prove erroneous at a subsequent instant because of the mobility of the communicating entities. This error is qualified in relation to a relative movement defined as a function of the speed of displacement of the source communicating entity observed from the destination communicating entity. For significant relative movements, there is a decorrelation between the estimate of the propagation channel, applied for the pre-distortion of the signal, and the propagation channel actually experienced by the signal.
Thus, on the one hand, pre-distortion of the signal is inadequate and, on the other hand, the focusing of the signal on a destination antenna is not achieved. The technique of pre-equalization by time reversal therefore gives poor performance for communicating entities that are moving rapidly.
The defocusing of the signal is in fact caused by the relative movement but also by a latency delay. The latency delay is the overall delay comprising a processing delay between the measurement of the propagation channel and the transmission of the focused signal, and a propagation delay between the transmission and the reception of the focused signal.
The position of the destination antenna relative to the estimation of the propagation channel applied to the pre-equalized signal defines a focal point. The focal point thus corresponds to the position of the destination antenna relative to the source antenna before the latency delay has elapsed. In other words, the focal point corresponds to the point of convergence of the waves for static communicating entities and in the absence of latency delay. In the case of a displacement of one or both communicating entities during the latency delay, the destination antenna is at a position that differs from the position of the focal point relative to the source antenna, therefore the focal point is not observed.
When the focusing of the signal is not achieved, the service quality is degraded. The service quality is, for example, the bit rate offered or else an error ratio on the data conveyed by the signal. Thus, the service quality is degraded as soon as the destination communicating entity and/or the source communicating entity moves. This degradation increases when the relative movement of the source communicating entity relative to the destination communicating entity increases, this movement occurring during the latency time. In other words, the power received on the destination antenna decreases when the relative movement increases during the latency time. Furthermore, this degradation is a function of the carrier frequency of the pre-equalized signal.
It is therefore necessary for the source communicating entity to adapt transmission parameters relating to the focusing of a signal, notably as a function of the relative movement of the communicating entities, in order to guarantee a service quality that may be, for example, a bit rate offered or else an error ratio on the data conveyed. The transmission parameters are, for example, the latency delay or the carrier frequency of the transmitted signal. The communicating entity can also choose to change bandwidth, and therefore carrier frequency, within one and the same radio communication system, or else to switch over to a separate radio communication system operating in a separate bandwidth.
There is therefore a need for a method for dynamically controlling the focus of a signal pre-equalized by time reversal in order to adapt the transmission parameters of a signal pre-equalized by time reversal.